The present invention relates to a method of and an apparatus for continuously or semi-continuously casting rectangular metal ingots, and more particularly to a method with which it is possible to produce ingots of rectangular cross section from light metals, particularly aluminum or aluminum base alloys, with consistently high quality.
Numerous methods have been proposed in the art for continuous or semi-continuous casting of metal ingots from metals such as aluminum and aluminum base alloys. A typical example of such methods is disclosed in the U.S. Pat. No. 2,983,972 in which a vertical open-ended casting mold is closed at its lower open end by a stool which initially forms the bottom of the mold but is lowered as molten metal is poured into the mold cavity through a nozzle or a trough. As the stool is progressively lowered in step, a column of liquid metal within the mold initially cooled by contact with the inner wall of the mold which contains a coolant (usually, water) circulating through a passageway formed therein, is then cooled directly by a splash of the coolant delivered through a slit in the lower end of the mold, whereby the liquid metal column is solidified as it emerges from the mold. Thus, the intended solid ingot is continuously formed and withdrawn from the mold.
It has been frequently recognized that the continuous production of metal ingots with such conventional methods may result in ingots having various sorts of surface defects or irregularities which are referred to as "liquation", "cold shuts", "stickings or weldings", etc. Developments of these defects which have adverse effects on the end products (obtained by processing the ingots) need to be reduced to a practical minimum.
It has already been observed that the development of the above surface defects are result from a thin layer of solidification shell in the mold which is formed by a primary cooling thereof with a coolant circulating within the mold to cool the inner wall. In view of this observation, it has been conventionally suggested to prevent formation of such thin weak layer of the solidification shell for the purpose of improving surface quality of the ingots. One of such remedies for removing or reducing the surface defects is the use of a hot top casting process, as typically shown in the U.S. Pat. No. 3,612,151, wherein an insulated feed reservoir is axially aligned with a mold and the casting speed for a metal is established so that the upstream conduction distance measured from the liquid wetting line of the coolant on the ingot surface extends to within about 1 inch of the reservoir, in order to substantially eliminate a thin layer of solidification shell formed within the mold and conduct a casting operation so that only a rigidly solidified metal is formed through direct cooling of the liquid metal (secondary cooling) by the coolant discharged from the bottom of the mold. Another casting process similar to the above hot top process has been proposed, as disclosed in the U.S. Pat. No. 3,326,270, wherein an upper part of the mold wall is covered with a tubular heat-insulating member with its lower end located at a predetermined position on the mold wall so that the front of solidification by secondary cooling (direct chilling) is located just beneath the lower end of the heat-insulating member.
Although it has been recognized that the above conventional solutions to the development of surface defects on the ingot surface are effectively applied to the production of ingots of circular cross section, it has been extremely difficult to apply those solutions to the production of ingots of rectangular cross section with improvements of surface quality as much as attained where the ingots to be produced are circular in cross section. In casting an ingot of rectangular or square cross section, the level or line of solidification by direct chill or secondary cooling is different at different positions on the periphery of the rectangular mold. More specifically, the solidification level is higher at the corners of the inner wall surfaces of the mold than at the central portions between the corners. Therefore, a mere shifting of the level of solidification by a constant amount at all portions of the ingot will not lead to simultaneous elimination of the previously indicated weak brittle layer of solidification shell in the mold from all portions of the ingot adjacent to the inner periphery of the mold. Thus, the mere use of the heat insulators on the inner mold wall has not been successful in attaining consistent improvements in the surface quality over the entire periphery of the ingot.
Whereas, one of the present inventors proposed a method of continuously casting metal ingots, which is the subject matter of the Japanese patent application TOKUGAN-SHO No. 51-91719 (laid open as TOKU-KAI-SHO No. 53-16323), in which a suitable open-ended heat-insulating member of rectangular cross section is disposed so as to be interposed between an upper part of the inner mold wall and the outer periphery of molten metal while means for controlling thermal conductivity of the mold wall is provided, in order to eliminate the previously indicated weak brittle layer of solidification shell. The disclosure of the above application clarifies the foregoing problem encountered in the casting of rectangular ingots, i.e., difference in formation of the solidification shell between the central portion of each side of the rectangular cross section of the ingot and the corner portions at the end of each side, and the same disclosure also clarifies that the above problem is favourably solved by the use of a heat-insulating member whose four sides each comprise a central portion which projects downwardly of the mold wall.
However, further analysis and observations of the casting process of rectangular cross sectional ingots by the present inventors dictated that the above proposed heat-insulating member with improved lower end profiles are not completely satisfactory for sufficient improvements in the surface quality and skin structure of the ingots, and clarified that there still exists a problem of difficulty in obtaining a consistent surface finish throughout the periphery of the ingots. Thus, there has been a requirement for further improvement in the process for casting rectangular ingots.
Through intensive research and investigation in view of the above situation, the inventors obtained a finding that the foregoing problems experienced in the art can be more effectively overcome, that is, an ingot of rectangular cross section can be cast with consistently high surface quality throughout the periphery thereof, by means of properly positioning or dimensioning a heat-insulating sheet which is interposed between an upper part of each inner wall surface of the mold and the outer periphery of molten metal. More particularly stated, each of the heat-insulating sheets which prevent direct contact of the poured molten metal with the wall surfaces on four sides of the mold, is adapted such that its central and horizontal end portions covering the respective central and end (corner) portion of each side of the rectangular mold are dimensioned or their lower end profiles are determined to satisfy a predetermined relationship according to casting conditions and specific kinds of metals to be cast, and such that the central portion of the sheet is downwardly projected beyond the adjacent end portions and has a straight lower end profile extending horizontally over a predetermined distance from the center of the respective side of the mold toward the end portions.